A quantum well resonator comprises a thin layer of a narrow bandgap semiconductor between two thin layers of a wider bandgap semiconductor. The layer of narrow bandgap semiconductor is sufficiently thin that the layers of wider bandgap semiconductor perturb the energy band structure of the narrow bandgap semiconductor, resolving the energy bands into discrete energy levels.
In known two terminal semiconductor devices which rely on resonant tunnelling for their operation, one or more quantum well resonators are located between two regions of narrow bandgap semiconductor. When an electrical bias is applied across the quantum well resonator by means of electrical contacts to the regions of narrow bandgap semiconductor, the quantum well resonator acts as a potential barrier inhibiting the flow of carriers between the electrical contacts unless the energy of carriers on one side of the quantum well resonator corresponds to one of the discrete energy levels within the quantum well resonator. When the energy of carriers on one side of the quantum well resonator corresponds to one of the discrete energy levels, however, carriers flow through the quantum well resonator by a mechanism known as resonant tunnelling. Thus, the current-voltage characteristics of two terminal resonant tunnelling devices have current peaks at bias voltages which raise the carrier energy at one side of the quantum well resonator to an energy which corresponds to one of the discrete energy levels within the quantum well resonator and troughs between such bias voltages.
Quantum well resonators have been included in bipolar and unipolar transistors to provide three terminal devices which rely in part on resonant tunnelling for their operation. For example, the Resonant Hot Electron Transistor (RHET) is a unipolar transistor which includes a quantum well resonator located between an emitter region and a base region of the bipolar transistor. The quantum well resonator inhibits injection of carriers from the emitter region into the base region unless the emitter-base bias is favourable for resonant tunnelling.
The Resonant Tunnelling Bipolar Transistor (RTBT) is a bipolar transistor which includes a quantum well resonator located in a base region of the bipolar transistor. In this device, carriers are thermally injected from an emitter region into the base region. The quantum well resonator inhibits transport of the injected carriers across the base region to a collector region of the bipolar transistor unless the emitter-base bias is favourable for resonant tunnelling.
The BIpolar QUAntum Resonant Tunnelling Transistor (BIQUARTT) is a bipolar transistor which, like the RTBT, includes a quantum well resonator located in the base region of the bipolar transistor. Unlike the quantum well resonator of the RTBT however, the quantum well resonator of the BIQUARTT includes doping junctions between the layer of wide bandgap semiconductor and the layer of narrow bandgap semiconductor which define the quantum well resonator. These doping junctions permit isolated electrical contact to the narrow bandgap layer of the quantum well resonator, so that the discrete energy levels in this layer can be controlled directly by application of an electrical bias. Control of the discrete energy levels permits control of resonant tunnelling through the quantum well resonator. Unfortunately, it is very difficult to form very closely spaced distinct doping junctions as are required for the BIQUARTT and the narrow bandgap layer of the quantum well resonator must be so thin that an electrical contact to that layer has a very high resistivity.
The Resonant Tunnelling Field Effect Transistor (RTFET) is a Field Effect Transistor (FET) which includes a quantum well resonator between a gate of the RTFET and a channel extending between a source and a drain of the RTFET. The RTFET operates as a conventional FET when the gate-drain bias is unfavourable for resonant tunnelling between the gate and the channel. However, when the gate-drain bias is favourable for resonant tunnelling through the quantum well resonator, the drain current is augmented by the tunnelling current.
Copending U.S. patent application Ser. No. 288,581 filed Dec. 22, 1988 in the name of Derek J. Day describes a family of electronic and optoelectronic devices having a pn junction which intersects a heterojunction. The materials and doping of the devices are selected to induce formation of a two dimensional minority carrier gas extending from the pn junction along the heterojunction on one side of the pn junction. The population and extent of the two dimensional minority carrier gas is modulated by biasing the pn junction to drive carriers across the pn junction, by applying an electric field across the heterojunction to modify a confinement potential for the minority carriers at the heterojunction, or by optically irradiating the pn junction to generate carriers for injection into the minority carrier gas. The two dimensional minority carrier gas modulates a current conducted along or across the heterojunction and can be used as a source of minority carriers for optical recombination. The family of devices includes electronic amplifying and switching devices, optical sources and detectors, and electronic switches which are sensitive to optical inputs. The specification and drawings of U.S. patent application Ser. No. 288,581 are hereby incorporated by reference.